1. Field of the Invention
The present invention relates to a method and an apparatus for preventing tag collision in a Radio Frequency IDentification (RFID) system. More particularly, the present invention relates to a method and an apparatus for predating the number of tags based on receiving power and allocating slots at a reader in the RFID system.
2. Description of the Related Art
A Radio Frequency IDentification (RFID) system is a system which provides an automatic identification method, using a barcode, a magnetic sensor, or an Integrated Circuit (IC)-card, for wirelessly retrieving data stored in microchips using microwaves or long waves. Because of its advances, the RFID technology is regarded as a replacement for the simple barcode and its use is increasing in industry.
To retrieve the data stored in the microchips, the RFID system does not need to contact a reader directly to the microchips, or to separately scan the microchips in sight using the reader. Also, the RFID system features mass data transmission.
Tag collision preventing methods of the RFID system include a deterministic method and a stochastic method. A frame slot Aloha of the stochastic method is being used.
The frame slot Aloha is a channel multiple access technology where a wireless communication device communicates data frame by frame constituted with time slots, which are regular time intervals, using a channel when a plurality of wireless communication devices sharing a single channel intends to transmit data.
With a relatively simple implementation, the frame slot Aloha may be widely adopted in wireless communications and applied to the RFID system.
When the RFID system adopts the frame slot Aloha, a response time of an RFID tag is divided into the fixed-size slots in response to a transmission command of an RFID reader, the multiple RFID tags transmit their ID number in their selected slot, and the tags are identified by constituting one frame with the multiple slots.
FIG. 1 is a diagram illustrating conventional frame slot Aloha operations.
In the basic frame slot Aloha, a frame size used in communications between the RFID reader and the RFID tags is fixed. When the RFID reader broadcasts an ID number request message to the RFID tags, the message carries information relating to the frame size and slot status to the RFID tags.
The RFID tag selects a slot to use in the frame using the information relating to the frame size and the slot status received together with the ID number request message.
Referring to FIG. 1, when the ID number transmitted by the RFID tag is received at the RFID reader without collision, the corresponding tag is identified. Accordingly, the corresponding tag is commanded not to respond to subsequent ID request messages.
Since the frame of FIG. 1 has a read cycle including three slots, the frame size is fixed. The RFID reader sends the ID number request message to five RFID tags.
Within the first read cycle, a first tag 101 and a third tag 103 send their ID numbers in a first slot 111, and a second tag 102 and a fifth tag 105 send their ID numbers in a second slot 112. In this situation, the tags collide with each other. As recognizing only a fourth tag 104 in a third slot 113 in the first read cycle, the RFID reader commands the fourth tag 104 not to respond to next ID number request message.
Since the first tag 101, the second tag 102, the third tag 103, and the fifth tag 105 are not identified in the first read cycle because of the tag collision, they randomly select a slot in the second read cycle and respond to the second ID number request message of the RFID reader. In the second read cycle, the second tag 102 and the fifth tag 105 have tag collision in a first slot 121. The first tag 101 is recognized in a second slot 122 and the third tag 103 is recognized in a third slot 123.
The basic frame slot Aloha identifies the RFID tags as stated above. When a small-size frame is used for a great number of tags, the tag reading efficiency drastically falls down because of the tag collision even through the multiple repetitions of the read cycle. When a large-size frame is used for a small number of tags, the slots are unnecessarily wasted.
To address those problems, a dynamic frame Aloha method for changing the frame size and a tag number estimation frame Aloha method have been suggested.
The dynamic frame Aloha method has many modified algorithms depending on its implementation. One of the modified algorithms increases the frame size when the number of colliding slots exceeds a certain ratio of the frame, and decreases the frame size when slots over a certain ratio are empty in the frame.
The tag number estimation Aloha method identifies tags by estimating the number of tags in the read range of the RFID reader and determining an optimum frame size. Using the slot status based on the tag collision after the read cycle, this method determines the frame size to be applied to the next read cycle.
The dynamic frame Aloha method and the tag number estimation Aloha method can enhance the tag reading efficiency by adjusting the frame size, compared to the basic frame Aloha method.
However, when the number of slots allocated by the RFID reader is greater than the number of the tags, many slots without response cause a waste of read time and power of the RFID reader. When the number of the slots allocated by the reader is smaller than the number of the tags, the tag responses collide with each other and the time and the power of the RFID reader are wasted in the re-identification.
As discussed above, to stably determine the slots, the two conventional methods are subject to the multiple frames.